1. Field of the Invention
The invention relates to an apparatus for controlling the flow rate of cementing fluid pumped into a well casing to effect the cementing of the casing in a well bore by flow downwardly through the casing, out the bottom end of the casing and upwardly around the casing.
2. Summary of the Prior Art
Substantially every cementing operation faces the problem of cement "free fall". The densities of commonly used cementing fluid substantially exceed that of well fluids and drilling mud which are present within the casing and in the annulus between the exterior of the casing and the well bore at the beginning of the cementing operation. As a result, when a cementing fluid is pumped into the well casing, the hydrostatic pressure on the well fluids at the bottom of the casing is significantly increased, causing a large rate of return flow of the well fluids upwardly around the casing, such rate of flow being substantially in excess of the flow rate of the cement being introduced into the casing. As a result, the imbalance between the densities of typical cements and muds leads to a period where the heavy column of cement in the casing "falls" away from the surface and that creates one or more void spaces in the upper portion of the casing. As a result, the surface lines are "on vacuum" and the pumps are pumping against zero gauge pressure. The return rate exceeds the pump rate part of the time and then is less than the pump rate in the later portion of the cementing operation.
The cement "free fall" condition can be aggravated by a much higher weight of cement contained in the casing bore compared to the total weight of mud, well fluids and little or no cement contained in the well bore annulus.
The condition of free fall leads to the following potential problems:
1. Any air sucked into the casing by leakage during free fall periods lead sto frothy cement;
2. High evaporation/low heat loss in cement under vacuum conditions can lead to dehydration of the cement;
3. Lower internal pressure while the vacuum pockets exist leads to higher collapse differential on the casing;
4. Water hammer results when pumping catches up with the cement column, and can cause damage to casing or surface equipment; and
5. Because the annulus flow rate can be higher or lower than the pump rate, this can result in non-turbulent flow during low flow rate periods while higher annulus flow rates lead to additional back pressure on production formations.
While the fre fall problem has existed for many years, the industry has yet to develop a satisfactory solution for eliminating or significantly reducing this problem.
The present invention is directed to each of these problems.